Holley Two-Barrel Carburetor Tunable Metering Block

1/6What was once a custom tuner part is now available off the shelf. Holley's tunable metering block (left) fits its PN 080787-1 HP 2-bbl carb (right) and offers full adjustability of the carb's fuel curve previously only available from specialists.

In the not too distant racing past racers had to build some of the heavy-duty parts they needed from scratch, or customize existing ones to make them suitable for racing. Today, racing parts that were once only available from your or your tuners' ingenuity are now mass-produced and ready off the shelf for even the most entry-level racing.

A case in point is this adjustable metering block derived from the stock metering block for Holley's PN 7448-350 two-barrel (2-bbl) carburetor. This unit was originally designed as a 350-cfm Ford replacement carb for street applications, but it is used nationwide in weekly racing series requiring a 350-cfm 2-bbl. Now, a fully tunable metering block that was once only available as an exotic drilled piece from a custom carburetor tuner or engine builder is offered as a retrofit metering block straight from Holley as PN 134-276. It's been designed particularly for its Keith Dorton Signature Series PN 0-80787-1 HP, a race-ready 350-cfm 2-bbl carburetor. In fact, all new Holley PN 080787-1 HP 2-bbls are being shipped with this tunable metering block already installed.

2-bbl Metering Block Origin While carb and engine tuners do work their tuning trade on the main body of a Holley carb for gains, it's the fine adjusting of the metering block that unlocks power and driveability from the carb. Plenty of professional and amateur carb tuners have taken their drill bits to the holes in the metering block in search of performance. For instance, Circle Track Tech Council member Keith Dorton originally worked with Holley to develop and test a tunable metering block with adjustable screw-in orifices (bleeds) for its 390-cfm, 4-bbl; it was sold as a separate part number and used in top touring series racing.

2/6This cutaway shows the positioning of the five screw-in emulsion air bleeds for the main circuit of the carb. A tuning "bleed kit" (PN 36-322) contains 10 each of these six other bleed sizes: Blank/solid, 0.020-inch, 0.031-inch, 0.040-inch, 0.046-inch, and 0.052-inch. The blanks are for drilling custom hole sizes.

That extensively adjustable metering block didn't go unnoticed by other engine builders. Indeed, expert Late Model engine builder Darrell Poe of Bear Creek, North Carolina, wanted to see how this 4-bbl adjustable metering block could be adapted to a 2-bbl carb-the Holley 7448 he's required to run on the NASCAR Late Model Stock engines he's noted for. Poe has a True Value Mechanic of the Year National Champion award from 2000, and six yearly regional True Value Mechanic awards, including one in 2002, to attest to how highly his peers deem his capabilities. He started to experiment with the 4-bbl adjustable metering block in the fall of 2001, and showed one of these production-modified metering blocks from Holley for the 390 4-bbl to NASCAR Weekly Racing officials to get a preliminary ruling on its legality with a 2-bbl. Plenty of discussion followed.

They rejected it outright for weekly series racing, even though the Holley factory part had been available for about a year for BGN (Busch Grand National)-level engines. In fact, Poe took some heat from fellow competitors, being accused of this extensive metering block modifying, even though it came from the factory that way. This was sort of a backhanded compliment. Holley's racing field tech representative Bobby Writesman clarified to the NASCAR Weekly Racing powers that Poe was in fact offering up a factory-produced part for the Dorton series 390 4-bbl. This was the initial contact between Holley and Poe, which led to his help in the R&D of a 2-bbl version of the tunable metering block and eventual acceptance. The adjustable 2-bbl metering block became eligible for use in NASCAR's Weekly Racing Series for Late Models beginning in 2002.

What's Adjustable? The Holley factory tunable 2-bbl metering block is easily identified by the numbers "12323" stamped into the casting on its throttle lever side. It comes complete with mixture screws, power valve, jets, gaskets, vent whistle, and a complete set of emulsion bleeds, including power valve channel restrictions, and a high-rpm "anti-siphon bleed" or "kill bleed" for the accelerator pump passage on the jet side of the block.

3/6The five screw-in emulsion air bleeds on each side of the metering block are sized from the factory beginning at the bottom bleed to top one: 0.031-inch, 0.031-inch, 0.031-inch, 0.020-inch, and 0.020-inch. They give a tuner full adjustability to the 2-bbl's fuel curve to fit any engine brand and combination.

Emulsion Air Bleeds For starters, the stock 7748 2-bbl metering block doesn't have emulsion tube holes drilled in it. According to Poe, "Carburetor tuners and engine builders trying to alter the fuel curve started playing around with the emulsion tube channels. They drilled different sized holes to introduce and mix air with the fuel, and to get it coming up into the booster to gain performance. You're ultimately trying to get the engine to operate at say 12.5:1 air/fuel ratio throughout its power curve, from top to bottom. Right out of the box, originally, the stock 2-bbl body and metering block do exactly the opposite of what we want them to do in a racing engine: It runs real lean down low (rpm) and real rich up top."

When you introduce air to this fuel channel, you're emulsifying the fuel-you're introducing air in different volume amounts (controlled by the number of bleed holes and their diameter), and at different points in the fuel curve (controlled by their vertical placement in the channel)-to alter the fuel curve and maintain the best air/fuel ratio for your engine combination. The tuning goal is to optimize the quality of the air/fuel mixture coming from the booster throughout the power band. The Holley 2-bbl tunable metering block has five adjustable emulsion screw-in bleed holes, and generally the largest size bleed will be at the bottom of the main well and the smallest at the top.

Unfortunately, a major problem with modifying these fuel channels with a drill on a stock Holley metering block is that once you make a hole that turns out to be too big and hurts performance, it's very difficult to plug the hole and start over with a smaller hole. You can quickly have a large collection of useless "experimental" metering blocks. That's why Holley offers various size screw-in emulsion bleed jets from 0.000-inch (blank)-0.078-inch for all the tunable orifices in this 2-bbl adjustable metering block.

4/6The PN 134-276 metering block comes with a 45 power valve (removed) and two 0.042-inch screw-in power valve channel restrictions (pointer). They allow more precise control of the fuel volume delivered to the engine when the power valve is open.

Power Valve Channel Restrictions The power valve lets additional fuel in when manifold vacuum drops under hard acceleration, such as when accelerating off a corner. The new adjustable fuel jets or "power valve channel restrictions" in the power valve mounting base in the adjustable metering block now allow tuning of the volume of fuel delivered, not just the timing of it. In a stock metering block, only the timing of the power valve actuation can be tuned by changing out the complete power valve to one of a different manifold vacuum rating. The number stamped on a power valve, such as "65," shows the manifold vacuum below which a power valve works/opens. In this case, a 65 means that, at all manifold vacuum below 6.5-inch Hg (Mercury), the power valve is working. A power valve times when fuel is delivered under acceleration-it is somewhat of a linear delivery, but it will flutter a little as it shuts off. The power valve in the adjustable 2-bbl metering block is stamped "45."

Poe explains the tuning utility of the adjustable power valve channel restrictions. "On the dyno, you can pull an engine and in the mid-range (say from 4,000-5,500 rpm and without changing the main jet-which can alter the fuel curve over the entire power band), you can adjust the power valve fuel bleeds and affect the fuel curve just in this mid-range band. That's a useful tuning aid." Changing the diameter of the power valve channel restrictions can lean or richen the desired fuel curve at the time the power valve opens. The tunable 2-bbl metering block comes with 0.042-inch screw-in restrictions.

5/6This anti-siphon air bleed breaks the siphoning vacuum pulled on the accelerator pump passage. It stops unmetered fuel from being sucked into the venturis from the pump discharge nozzles at WOT and richening the mixture. It comes stock at 0.020-inch. Note the reinforcing ribs of the adjustable metering block to add strength.

Anti-Siphon Air Bleed Box-stock, the non-adjustable 2-bbl metering block has a fuel delivery quirk that is a race engine-tuning nightmare. At WOT (wide-open throttle), with high vacuum caused by the high velocity of the air in the venturis, fuel is pulled out from the accelerator pump discharge nozzles-in two unmetered solid streams. Not exactly the optimum condition for managed fuel metering required for racing, is it? The total air/fuel mixture is richened without control by this siphoning effect. Consequently, an anti-siphon air bleed is placed in the accelerator pump passage just above the main jets in the tunable 2-bbl metering block. Air introduced into the pump passage by this screw-in air bleed (stock is a 0.020-inch diameter hole) breaks the siphoning vacuum being pulled on the fuel, and stops the fuel from being sucked up into the venturii at high vacuum/WOT. A smaller size hole will have less effect on the fuel suction, and a larger one will have more influence.

Dyno-tune, Not Seat-of-the-Pants The tunable metering block makes the Holley 0-80787-1 2-bbl carb more usable. For example, there are many different engine combinations racing in the NASCAR Late Model series, and since Dodge came into the series with yet another engine block and cylinder head combination (see Circle Track Oct. '02 issue for an exclusive full description of their Late Model engine), a one-tune-fits-all carb was unrealistic.

According to Poe, "The carb needs to be tuned to the Ford, Chevy, or the Dodge, whichever it's on. It needs to be more accurately tuned if it's on a 5.7-inch rod engine with a greater duration camshaft, or a 6.25-inch rod engine, which is likely to have a much shorter duration camshaft." He continues, "This metering block, used on the class-legal 2-bbl carburetor, gives the adjustability to gain peak power out of any engine combination it's installed on."

6/6Expert Late Model engine builder Darrell Poe at the controls of the dyno he used to help develop the 2-bbl adjustable metering block. He strongly recommends tuning this versatile metering block with dyno runs, not just seat-of-the pants changes. He has six regional True Value Mechanic awards, so his is a voice of fuel-curve tuning experience.

But all this adjustability should not be taken lightly. "There are 19 holes in this metering block to be played with to make this carb work on any engine combination," clarifies Poe. "As long as Holley has a hole in the metering block, we can upsize it, or downsize it, but we can't add any more holes to it."

The PN 134-276 adjustable metering block gives a carb tuner or engine builder adjustability with 13 of these holes. The engine tuner should use a dyno to take full advantage of all the fuel metering adjustability to quantify any gains (or losses). Poe won't give up all his metering block wizardry, but he offers this tuning starting point: "The engine tuner/builder needs to calculate the amount of area now being added to the fuel channel. If there are now three drilled emulsion holes with 'X' amount of square inches of area (in their combined hole dimensions), and now there are five adjustable holes, then begin with the same amount of area spread out over the five holes now available." Essentially, duplicate the hole area in the adjustable emulsion holes of the new metering block to create a tuning baseline.

Poe says he's seen as much as "5 to 7" hp gain on a Chevrolet engine between a standard 7448 and the HP carb with its adjustable metering block. "In track testing, back-to-back, a good race 7448 as we used to run, compared to an HP with the adjustable bleeds, has yielded as much as 0.010 seconds or more. This was with a Late Model Stock with one of my Chevy engines," explains Poe.